This invention relates to a mechanism for adjusting the height of a chair back relative to the seat of the chair.
The invention has been devised in the context of back height adjustment mechanisms for office chairs. A chair of this type typically includes a seat supported on a base provided with castors. A generally L-shaped arm extends rearwardly from below the seat and then upwardly and the back is coupled to the upright limb of the arm so that the back can be adjusted height-wise on the limb to suit the comfort and preference of the user of the chair.
A simple form of adjustment mechanism includes a bolt having a head in the form of a large knob or knurled handle that can be turned manually by the person adjusting the chair to clamp the back to the arm. Adjustment is essentially a two-handed operation in that the bolt must be loosened while the back is supported and then moved to an adjusted position. The back is then held in that position while the bolt is re-tightened.
More recently, adjustment mechanisms have become available that can be operated easily using one hand. U.S. Pat. No. 5,649,741 (Beggs) and U.S. Pat. No. 5,685,609 (Miotto) both disclose adjustment mechanisms of this type in which a pawl carried by the chair back engages a rack on the upright limb of the arm. As the back is raised, the pawl ratchets along the rack until an appropriate adjusted position is reached. If the chair back is moved to the top of its travel, the pawl disengages and the back can be moved down to a bottom position, where the pawl re-engages and the adjustment process can start again.
An object of the present invention is to provide an improved chair back height-adjustment mechanism.
The mechanism provided by the invention includes first and second structural members that are adapted to be connected to the seat and back respectively and are coupled to one another for relative sliding movement along a defined axis along which the back is to be adjusted with respect to the seat. A rack is provided on the first structural member and extends in the direction of the defined axis. A pinion carried by the second structural member engages the rack so that the pinion can rotate and move along the rack in response to relative movement between the first and second structural members along the said axis. A pawl is pivotally mounted on the second structural member for movement between (a) a position in which the pawl engages the pinion and permits rotation of the pinion in one rotational direction only corresponding to relative movement between the first and second structural members for height-wise adjustment of the chair back in a first direction along said axis, while preventing relative movement between the first and second structural members in a second, opposite direction, and (b) a disengaged position in which the pawl is clear of the pinion and the first and second structural members are free for movement in either said direction. Actuator means is carried by the second structural member for moving the pawl between its engaged and disengaged positions at respective limits of travel of the first and second structural members with respect to one another along said axis.
Typically, the first structural member of the mechanism comprises an arm that extends upwardly from the seat of the chair and the second structural member is a carriage that rides on the arm and is coupled to the back. While the mechanism can be designed to operate in either direction, the arrangement preferably is such that the back is moved incrementally in the upward direction to find a suitable adjusted position. When the limit of travel in that direction is reached, the actuator means disengages the pawl from the pinion so that the back can be moved freely downwardly to the bottom limit of its travel where the pawl is re-engaged so that the adjustment process can start again.
The first structural member (arm) may be connected directly to the seat but normally will be connected to the seat via the chair base.
It is believed that a rack and pinion adjustment mechanism of the form provided by the invention offers a number of significant advantages over the prior art, particularly those exemplified by the Beggs and Miotto patents referred to previously. The use of a rack and pinion allows the use of a finer tooth pitch so that finer increments of adjustment are possible. Also, the mechanism can be designed to operate more quietly. A number of other practical advantages are offered, in terms of ease of assembly and lower cost.